With the miniaturization and increasing power of microelectronics, heat dissipation has become critical to the performance, reliability and further miniaturization of microelectronics. Heat dissipation from microelectronics is most commonly performed by thermal conduction. For this purpose, a heat sink, which is a material of high thermal conductivity, is commonly used. In order for the heat sink to be well utilized, the thermal contact between the heat sink and the heat source (e.g., a substrate with a semiconductor chip on it) should be good.
The proximate surfaces involved in a thermal contact are never perfectly flat. There are hills and valleys in the surface topography, thus resulting in air pockets, which are thermally insulating, at the interface. Since air is a thermal insulator, it is important to displace the air by using an interface material that conforms to the topography of the mating surfaces. Therefore, conformability is an essential attribute of a thermal contact enhancing interface material, which is also known as a thermal interface material. The importance of conformability is mentioned in PCT Int. Appl. WO 9741599 (1997).
A thermal interface material that is thick (i.e., the thickness above about 50 μm, typically above about 100 μm) is needed for filling the gap between the two proximate surfaces, in case that the surfaces are not in direct contact (as encountered when each surface is curved, so that gaps exist at parts of the interface, or when the two surfaces are not exactly parallel). This category of thermal interface materials is known as gap filling materials. They are to be distinguished from thermal interface materials that are thin (i.e., the thickness below about 100 μm, typically below about 50 μm)—ideally just thick enough to fill the valleys in the topography of the mating surfaces.
The thin type of thermal interface material is mainly in the form of a paste, which is known as a thermal paste. This paste comprises a base medium (i.e., the vehicle) and a filler that is thermally conductive. The filler is typically in the form of particles, as it is more difficult to make a workable paste that contains fibers (discontinuous) instead of particles.
The workability relates to the conformability, although the type of conformability required for a thermal paste is conformability to the surface topography in a fine (microscopic) scale, so that the extent of flow associated with conforming is small (Xu, Luo and Chung, J. Electron. Packaging 124, 188-191 (2002), which is hereby incorporated by reference in its entirety). In contrast, workability usually refers to the ability to flow or deform in a relatively coarse scale. Conformability in a fine scale is more challenging than that in a coarse scale. The viscosity, which is a measure of the resistance to flow, is commonly used to describe the rheology of pastes. However, akin to the workability, the viscosity relates to the ability to flow in a relatively coarse scale.
For any thermal interface material, the thicker it is, the more is the thermal resistance that it gives. Thus, a small thickness (ideally just enough to fill the valleys at the interface) is also important for a thermal interface material. For a thermal paste, a small thickness can be attained if the paste is highly spreadable. Hence, spreadability is the second criterion. The importance of spreadability is mentioned in U.S. Pat. Appl. Publ. US 20040060691 (2004).
A thermal paste is preferably thermally conductive, in addition to being conformable and spreadable. In fact, it is customary in the prior art to include in the formulation of thermal interface materials constituents that are high in thermal conductivity for the purpose of maximizing the thermal conductivity of the thermal interface materials.
Examples of constituents of high thermal conductivity are metal particles (e.g., nickel particles, zinc particles, copper particles, aluminum particles, silver particles, etc.), ceramic particles (e.g., boron nitride particles, zinc oxide particles, aluminum nitride particles, etc.) and diamond particles. Among these materials, diamond is the most conductive thermally.
Graphite, diamond and fullerenes are three categories of elemental carbon. Graphite and diamond differ in their chemical bonding, crystal structure, electrical conductivity and thermal conductivity. Graphite has a hexagonal crystal structure, whereas diamond has a cubic crystal structure. The carbon atoms in graphite are sp2 hybridized in their chemical bonding, whereas those in diamond are sp3 hybridized. In other words, the chemical bonding in graphite is partly metallic and partly covalent, whereas that in diamond is totally covalent. Graphite is an electrical conductor as well as a thermal conductor, whereas diamond is an electrical insulator and an exceptional thermal conductor. The thermal conductivity of diamond is much higher than that of graphite. Fullerenes are different from both graphite and diamond in that they are in the form of molecules. The most common form of carbon is in the graphite category. Although ideal graphite is completely crystalline in its structure, carbons in the graphite category can have various degrees of crystallinity. The higher is the degree of crystallinity, the higher is the thermal conductivity. Irrespective of the degree of crystallinity, the chemical bonding in carbons in the graphite category involves sp2 hybridization.
Carbon black is a carbon in the graphite category. It can be partially crystalline, in contrast to ideal graphite (referred to as graphite), which is completely crystalline. Due to the low degree of crystallinity, carbon black is low in thermal conductivity compared to ideal graphite. However, carbon black is one of the most inexpensive forms of carbon, as it is akin to soot.
Faming Zhuanli Shenqing Gongkai Shuomingshu CN 1715360 (2006) is a Chinese patent filed on Jun. 30, 2004, which is after the priority date of Jul. 9, 2003 of this application and is also after the inventor's first journal publication (Leong and Chung, Carbon 41(13), 2459-2469 (2003)) that discloses the present invention in the form filed on Jul. 9, 2003. This Chinese patent mentions the use of carbon black particles (1-5%) as a ductile additional filler in a thermal interface material that contains particles of high thermal conductivity. Carbon black is not listed as one of the choices of particles of high thermal conductivity in this Chinese patent.
Faming Zhuanli Shenqing Gongkai Shuomingshu CN 1715360 (2006) distinguishes carbon black from carbon nanospheres, and recommends carbon nanospheres (not carbon black) to be one of the choices of thermally conductive particles. Similarly, in Faming Zhuanli Gongkai Shuomingshu CN 1517426 (2004), carbon nanoballs are recommended for serving as thermally conductive particles in formulating a thermal interface material.
Due to its structure, a multiwalled carbon nanotube is thermally conductive along the axis of each nanotube. The level of thermal conductivity depends on the degree of crystallographic order in the nanotube. The higher is degree of crystallographic order, the greater is the thermal conductivity. The thermal conductivity and the high aspect ratio (i.e., the ratio of the length to the diameter) of a nanotube are attributes which make the nanotubes attractive. The combined use of multiwalled carbon nanotubes (2%) and carbon black (10%) as thermally conductive constituents in a thermal interface material is recommended by Zhang et al., Proceedings—Electronic Components & Technology Conference (2005), 55th (Vol. 1), 60-65, published by Institute of Electrical and Electronics Engineers.
Due to its relatively low thermal conductivity, carbon black has not been suggested in the art prior to the priority date of Jul. 9, 2003 of this invention for use as a constituent in the formulation of a thermal interface material. Furthermore, carbon black has not been suggested in the art prior to the priority date of Jul. 9, 2003 of this invention for use in a thermal interface material for enhancing the conformability or spreadability of the interface material. In addition, carbon black has not been suggested in the art prior to the filing date of Jun. 28, 2006 of the present continuation-in-part application for use as the main thermally conductive constituent in a thermal interface material.
Due to the high thermal conductivity along the axis of a carbon nanotube, various forms of thermal interface materials involving carbon nanotubes have been reported. They include a carbon nanotube paste (PCT Int. Appl. WO 2006048848 (2006)), a carbon nanotube paraffin-matrix composite (U.S. Pat. Appl. Publ. US 20060073332 (2006)), aligned carbon nanotubes (U.S. Pat. Appl. Publ. US 20050255304 (2005)), a carbon nanotube array in a polymer matrix (US. Pat. Appl. Publ. US 20040097635 (2004)), carbon nanotubes aligned by using an alignment material such as a clay material (PCT Int. Appl. WO 2005031864 (2005)), a carbon nanotube array affixed to the surface of a thermal interface material layer (U.S. Pat. Appl. Publ. US 20040184241 (2004), carbon nanotubed distributed in a silver paste (Faming Zhuanli Shenqing Gongkai Shuomingshu CN 1667821 (2005), and an array of carbon nanotubes with silver filled therein (U.S. Pat. Appl. Publ. US 20060118791 (2006) and Faming Zhuanli Shenqing Gongkai Shuomingshu CN 1632040 (2005)). The high cost of the carbon nanotubes, the high cost of making carbon nanotube arrays, the geometric limitations of the arrays and the limited choices of array substrates are disadvantages that make practical use of these nanotube technologies difficult.
Due to its high thermal conductivity, boron nitride is used as a thermally conductive constituent in thermal interface materials. Compared to boron nitride, zinc oxide is less thermally conductive, but it is less expensive. Zinc oxide (ZnO) is the thermally conductive constituent of choice in the thermal interface material formulation in U.S. Pat. No. 6,475,962 (2002). Boron nitride (BN) is the thermally conductive constituent of choice in the thermal interface material formulations in U.S. 20040241410 (2004) and U.S. 20040081843 (2004).
Boron nitride suffers from the disadvantage that it degrades when exposed to humidity. When placed in a humid environment, hygroscopic impurities (boric oxide) within the compound absorb atmospheric water, which then reacts with the boron nitride to form boric acid. Being hygroscopic, the boric acid absorbs further water, thereby accelerating the degradation of the boron nitride and diminishing its heat removing capabilities, which ultimately leads to failure of the device. The PCT Application WO 01/21393 is specifically directed to this problem and describes a moisture resistant, thermally conductive material that includes thermally conductive filler particles, preferably boron nitride, that are coated with a hydrophobic compound, preferably a silicone compound such as a siloxane. The hydrophobic compound-coated filler particles are joined together with a binder, and account for between 5 and 70 vol. % of the material.
The vehicle of a thermal paste is typically not a thermal conductor. The thermal conductivity of the vehicle is typically much lower than that of thermally conductive fillers. Thus, the higher is the proportion of thermally conductive constituent in a thermal paste, the higher is the thermal conductivity of the paste.
For the purpose of maximizing the thermal conductivity of a thermal paste, it is customary in the prior art to use thermally conductive constituents at high proportions. For example, zinc oxide in the amount of 72.8% is used in the formulation in U.S. Pat. No. 6,475,962 (2002), boron nitride in the amount of 30% is used in the formulation in U.S. 20040081843 (2004), boron nitride in the amount of at least 25% is used in the formulation in U.S. 2004241410 (2004), and carbon black in the amount of 10% (along with multiwalled carbon nanotubes) is used in the formulation reported by Zhang et al., Proceedings—Electronic Components & Technology Conference (2005), 55th (Vol. 1), 60-65, published by Institute of Electrical and Electronics Engineers.
Both thermal conductivity and conformability help the performance of a thermal paste. The workability and conformability of a thermal paste diminish with increasing conductive filler content, although the thermal conductivity of the paste increases with increasing filler content. Thus, the use of an excessively high filler content results in low conformability, though the thermal conductivity is enhanced. Prior work on the development of thermal pastes has emphasized the attainment of a high thermal conductivity in the paste by using very high contents of conductive fillers. The low conformability that results from the high filler content has been of relatively little concern in prior work. Thus, prior work has emphasized the development of a workable paste that has a high content of the conductive filler. The approach in prior work largely involves the use of surfactants and the treatment of the surfaces of the filler particles (e.g., BN, ZnO, Al2O3, etc.) for the purpose of improving the workability, and the use of particles of different sizes in the same paste for the purpose of increasing the filler content.
Thermal conductivity has long been assumed in the thermal interface material industry to be the key criterion in determining the effectiveness of a thermal interface material, but it is actually less important than conformability or spreadability, as unexpectedly found in the present invention. As long as the thermal interface material is more thermally conductive than air, its presence can improve the thermal contact. On the other hand, if the thermal interface material is relatively large in thickness, its thermal conductivity can be important. Thus, this invention emphasizes thermal pastes that are small in thickness (less than 50 μm, typically less than 25 μm) during use after application, in contrast to prior work, which emphasizes thermal pastes that are much larger in thickness (above 50 μm, typically above 100 μm).
The conformability of a thermal paste also depends on the vehicle, i.e., the matrix. Silicone is the most commonly used vehicle, in spite of its high viscosity and the consequent low conformability and low spreadability. For example, U.S. Pat. Appl. Publ. US 20030171487 (2003) uses silicone and recognizes the high viscosity of the resulting thermal interface material. U.S. Pat. Appl. Publ. US 20050150887 (2005) and Faming Zhuanli Shenqing Gongkai Shuomingshu CN 1715360 (2006) also use silicone as the matrix.
During use, it is preferred that a thermal paste does not seep out of the interface, as the seepage can cause contamination and, in the case of an electrically conductive paste, short circuiting of the electronics around the thermal contact. Therefore, a thixotropic paste (a paste that flows only under an applied stress) is preferred to a fluidic paste (a paste that flows even in the absence of an applied stress). Silicone is thixotropic. Polyol ester can also be used to form a thixotropic paste, as described in U.S. Pat. No. 6,475,962 (2002) and U.S. Pat. Appl. Publ. US 20040018945 (2004).
Metal alloys with low melting temperatures (such as solders) applied in the molten state have long been used as thermal interface materials. However, they tend to suffer from the chemical reactivity of the liquid alloy with some metal surfaces (such as copper). The reaction products can interfere with the contact between the liquid alloy and a metal surface. Thus, although metals are high in thermal conductivity, they have limited conformability, thereby resulting in limited effectiveness as thermal interface materials. Furthermore, alloys suffer from the need to heat during their application. In contrast, thermal pastes do not require heating. Xu, Luo and Chung, J. Electron. Pkg. 124:188-191 (2002); Xu, Luo and Chung, J. Electron. Pkg. 122:128-131 (2000); both are hereby incorporated by reference in their entirety.
Organic vehicles are commonly used as the suspending medium for dispersed inorganic particles in pastes. Kumar, Active & Passive Elec. Comp. 25:169-179(2002); Chae et al., Mater. Lett. 55:211-216 (2002); Heller et al., Tenside, Surfactants, Detergents 29:315-319 (1992); Stanton, Int. J. Hybrid. Microelec. 6:419-432 (1983). An organic vehicle system may consist of a solvent (such as butyl ether) (Bernazzani et al., J. Chem. Therm. 33:629-641 (2001)) and a solute (such as ethyl cellulose) (Faming Zhuanli Shenqing Gongkai Shuomingshu CN 1715360 (2006); Stanton, Int. J. Hybrid Microelec. 6:419-432 (1983)), which serves to enhance the dispersion and suspension. Kumar, Active & Passive Elec. Comp. 25:169-179(2002). Ethyl cellulose has the further advantage of its slight conductivity. Khare et al., Polym. Int. 42:138-142 (1997); Khare et al., Polym. Int. 49:719-727 (2000).
Another organic vehicle is polyethylene glycol (PEG) of molecular weight 400 amu, which is different from silicone in its low viscosity. By using PEG in conjunction with boron nitride particles as a thermal paste between copper disks (of surface roughness 0.05 μm), a thermal contact conductance of 1.9×105 W/m2.° C. has been attained. This value is higher than that obtained by using a thermal paste involving silicone and boron nitride powder (1.1×105 W/m2.° C.), but is lower than that obtained by using solder, applied in the molten state (2.1×105/m2.° C.). Xu, Luo and Chung, J. Electron. Pkg. 124:188-191 (2002). In fact, all thermal pastes previously reported are inferior to solder in providing high thermal contact conductance. The use of PEG as the matrix is also mentioned in Faming Zhuanli Shenqing Gongkai Shuomingshu CN 1715360 (2006).
Carbon black is in a very fine particulate form and consists of typically spherical particles, which in turn come together to form porous agglomerates. Carbon black is produced either by incomplete combustion or thermal decomposition of a hydrocarbon feedstock. Types of carbon black include soot, lamp black (typical particle size 50-100 nm), channel black (typical particle size 10-30 nm), furnace black (typical particle size 10-80 nm), thermal black (typical particle size 150-500 nm), and acetylene black (typical particle size 35-70 nm).
Carbon black is used as a low-cost electrically conductive filler in polymers. Nakamura et al., NEC Res. & Dev. 83:121-127 (1986); Saad et al., J. Appl. Polym. Sci. 73:2657:2670 (1999). Most commonly, it is used as a reinforcement in rubber. Takirio et al., Tire Sci. & Tech. 26:241-257 (1998); Haws et al., Rub. Div. Symp., ACS, Akron, OH 1:257-281 (1982); Hess et al., Rub. Chem. & Tech. 56:390-417 (1983); Kundu et al., J. Appl. Polym. Sci. 84:256-260 (2002); Ramesan et al., Plas. Rub. & Comp. 30:355-362 (2001); Sridhar et al., J. Appl. Polym. Sci. 82:997-1005 (2001).
In addition, carbon black is used in electrochemical electrodes (Takei et al., J. Power Sources 55:191-195 (1995); Van Deraerschot et al., Electrochem. Soc. Ext. Abst., Electrochem. Soc., Pennington, N.J. 84:139 (1984)), inks (Erhan et al., J. Am. Oil Chem. Soc. 68:635-638 (1991); Bratkowska et al., Przemysl Chemiczny 66:393-395 (1987); Bratkowska et al., Przemysl Chemiczny 65:363-365(1986)), lubricants (Chinas-Castillo et al., Tribology Trans. 43:387-394 (2000); Shiao et al., J. Appl. Polym. Sci. 80:1514-1519 (2001); Kozlovtsev et al., Glass & Ceramics (English Translation of Steklo I Keramika) 154-157; Bakaleinikov et al., Chem. & Tech. Fuels & Oils 18:108-111 (1982)), fuels (Srivastava et al., Fuel 73:1911-1917 (1984); Steinberg, Preprints: Div. Pet. Chem., ACS 32:565-571 (1987); Smith, Automotive Eng. (London) 7:23-24, 27 (1982)), and pigments (Ueki et al., Ann. Conf. Elec. Ins. & Dielec. Phen., Ann. Rpt., IEEE, Piscataway, N.J. 1:170-176 (1997)).
A thermal interface material in the form of a sheet coated with a paste on both sides is attractive in that it can be conveniently handled like a sheet and that its thickness can be adjusted by adjusting the thickness of the sheet at its core. The sheet serves as a carrier for the paste. The ability to adjust the thickness is advantageous for applications in which the surfaces of the heat source and heat sink are locally apart from one another due to insufficient alignment, parallelism or flatness of the surfaces. Such thermal interface materials are known as thermal gap filling materials. A thermal interface material in the form of a copper mesh coated with a slurry that comprises a liquid metal alloy has been disclosed (PCT Int. Appl. WO 2005053021 (2005)).
Examples of sheets are fiber mats, wire meshes, fabrics, metal foils (such as aluminum foils and copper foils) and flexible graphite. A thermal interface material cannot be thermally insulating. Fiber mats, wire meshes and fabrics are porous. The thermally conductive paste can penetrate a porous sheet. Thus, a sheet does not have to be thermally conductive, if it is porous. Metal foils and flexible graphite are not porous, unless holes are made through them. However, they are thermally conductive.
“Flexible graphite” does not simply mean graphite that is flexible. It is a term that refers to a special form of graphite that is made by compression of exfoliated graphite flakes in the absence of a binder (Luo, Chugh, Biller, Hoi and Chung, J. Electron. Mater. 31:535-544 (2002); Luo and Chung, Carbon 39:985-990 (2001); Chung, J. Mater. Sci. 22:4190-4198 (1987), which are hereby incorporated by reference in their entirety).
The exfoliated graphite flakes used to make flexible graphite have an accordion microstructure. Due to the large expansion in the thickness of a flake upon exfoliation, an exfoliated flake is not of the shape of a flake, but is of the shape of a column, which is called a “worm” (due to the column shape and that the column can bend). The accordion microstructure allows the worms to mechanically interlock upon compression in the absence of a binder. Flexible graphite is a sheet that is flexible and is resilient in the direction perpendicular to the sheet. This resiliency results in some degree of conformability in the direction perpendicular to the sheet. This conformability is useful in relation to the use of flexible graphite as a thermal contact enhancing interface material, as is shown in the present invention.
Flexible graphite as a thermal interface material has been disclosed by Marotta et al., IEEE Trans. Compon. Pack. Tech. 28:102-110 (2005). Flexible graphite sheets containing oil have been disclosed in U.S. Pat. Appl. Publ. US 20050175838 (2005) and U.S. Pat. Appl. Publ. US 20030118826 (2003) for use as thermal interface materials.
Pressure applied to squeeze together the proximate surfaces with a thermal paste in between may or may not help the heat transfer across the interface, as shown by Xu, Luo and Chung, Journal of Electronic Packaging 122:128-131 (2000). The patent U.S. Pat. No. 6,472,742 (2002) teaches against the use of pressure.
The present invention is directed to overcoming these and other deficiencies in the art.